According to a press release by Apple, AFib, the leading cause of stroke, is responsible for approximately 130,000 deaths and 750,000 hospitalizations in the US every year. Many people don’t experience symptoms, so AFib often goes undiagnosed.

To calculate heart rate and rhythm, Apple Watch’s sensor uses green LED lights flashing hundreds of times per second and light-sensitive photodiodes to detect the amount of blood flowing through the wrist. The sensor’s unique optical design gathers signals from four distinct points on the wrist, and when combined with powerful software algorithms, Apple Watch isolates heart rhythms from other noise. The Apple Heart Study app uses this technology to identify an irregular heart rhythm.

“Every week we receive incredible customer letters about how Apple Watch has affected their lives, including learning that they have AFib. These stories inspire us and we're determined to do more to help people understand their health,” said Jeff Williams, Apple’s COO. “Working alongside the medical community, not only can we inform people of certain health conditions, we also hope to advance discoveries in heart science.”

Apple is partnering with Stanford Medicine to perform the research. As part of the study, if an irregular heart rhythm is identified, participants will receive a notification on their Apple Watch and iPhone, a free consultation with a study doctor and an electrocardiogram (ECG) patch for additional monitoring. The Apple Heart Study app is available in the US App Store to customers who are 22 years or older and have an Apple Watch Series 1 or later.

“Through the Apple Heart Study, Stanford Medicine faculty will explore how technology like Apple Watch’s heart rate sensor can help usher in a new era of proactive health care central to our Precision Health approach,” said Lloyd Minor, Dean of Stanford University School of Medicine. “We’re excited to work with Apple on this breakthrough heart study.”

Doctors and medical researchers around the world have been using iPhone and Apple Watch to revolutionize medical studies. Apps created with Apple’s ResearchKit platform, a software tool researchers use to conduct studies, have produced insights and discoveries about conditions like autism and Parkinson’s disease at a pace and scale never seen before. To date, Apple’s ResearchKit and CareKit platforms have been used by over 500 researchers and more than three million participants.